Cadherins are cell adhesion molecules important for the formation and organization of tissues, and their proper functions are necessary to prevent tissue degeneration in disease. Inside-out regulation of the adhesive function of cadherins in response to developmental and physiological signals plays a key role in controlling the dynamic interactions between cells during tissue morphogenesis. Although cytoplasmic signals controlling cadherins have been studied, very little is known about how the function of the extracellular adhesive domain is regulated; in part because the structure of the cadherin adhesive bond is still not entirely understood. An excellent model for elucidating the mechanism of cadherin inside-out regulation is C-cadherin in early embryos of the frog Xenopus laevis. Regulation of C-cadherin occurs in response to growth factors and is required for cell rearrangements and morphogenetic movements, called convergent-extension, that drive gastrulation and neurulation of the embryo. The crystal structure of the entire extracellular domain of C-cadherin has been solved and the homophilic adhesive binding properties of C-cadherin have been extensively analyzed by biophysical measurements; supporting several distinct molecular models of the homophilic bond. The major objectives of the proposed project are to elucidate the structure of the C-cadherin homophilic bond, to determine the molecular mechanisms underlying the insideout regulation of C-cadherin, and to develop tools that can be used to study the role of C-cadherin regulation in the control of morphogenetic movements in the embryo. The specific aims of the project are to: 1) determine the specific regions of C-cadherin that form the homophilic interaction sites; 2) determine which functional regions and protein interactions of C-cadherin are involved in the regulation of its adhesive activity in the Xenopus embryo; 3) use state or conformation specific antibodies as probes to analyze the mechanisms regulating the homophilic binding activity of C-cadherin; and 4) use quantitative fluorescence imaging to determine how the binding properties and organization of C-cadherin on the cell surface change during inside out signaling. These studies should help improve our understanding of the functions of the cadherins in tissue morphogenesis during embryogenesis and organ homeostasis, and the role of cadherin dysfunction in diseases such as cancer and birth defects.